(In reply to ISHIKAWA, Chiaki from comment #42)
So, again my conclusion is that the attachment download issue seems to be IMAP-specific?
That's probably the case, I'd guess:
With POP3, your server is only meant as a temporary "letter box". You check it regularly and take the emails "out" of the letter box and store them on your local device. Although it's possible to keep a copy of the emails on the server, POP3 is for people that want to have their emails on their device. When you open an email fetched by POP3 or an attachment of such an email, the email always already resides on your hard disk in your mbox/Maildir before you open it.
With IMAP, you want to keep your email storage on the server. You can benefit from the advantage of having your emails and folders in sync between different client devices (notebook, workstation, mobile phone) automatically. As the server-side storage is the "single source of truth".
Additional offline copies of your emails / folders are not necessary. But it can be useful to be able to work offline (on an airplane or something like that) or to speed up access. When you open an email or its attachments in this use cases, you might not have a local copy of the email (maybe headers only). So first you have to load the email from the central server storage, either into RAM or onto a local disk. If I get Glen right, Thunderbird only uses a limited amount of RAM (but not disk space) to load emails from the server if you don't have an offline copy. And this seems to fail for attachments larger than 20 MB if you don't manipulate the config.
In our case it's like that: We have a local Linux mailserver (Postfix + Dovecot) with RAID disks, automated nightly backups, etc. For us it's a better / safer place to keep the emails in this central storage than scattered on different client notebooks that can get lost, broken, corrupted due to user / usage errors, ...
As client devices, we use older laptops with Debian Linux and small SSDs (not larger than 128 GB) or smartphones (Android). For both types of devices, we had to stop keeping a local copy of the entire IMAP storage because the mailboxes are simply too large for the small disks. :(